The blood brain barrier (“BBB”) is formed by layers of cells lining the cerebral vasculature. As such, the BBB is able to maintain a stable environment in the brain by preventing the entrance of most substances such as toxins, drugs, viruses and bacteria from the blood stream into brain tissue. The blood brain barrier typically prevents trans-port of molecules larger than about 180 Daltons because of tight endothelial junctions (zonulae occludens), glial processes and basal lamina and the lack of fenestrations and transendothelial channels in the blood vessels' linings. It is also believed that permeability through the BBB may be further limited by the active transport of foreign substances out of the brain and into the lumen of the cerebral vasculature.
The BBB presents one of the largest obstacles to treating many brain diseases. Specifically, the BBB prevents many therapeutic agents, such as drugs and gene-therapy vectors, from accessing a patient's brain tissue. For example, infections of the central nervous system, neurodegenerative diseases, congenital enzyme defects and brain cancer are all affected by the ability of the BBB to block passage of, inter alia, antibiotics, anti-retroviral drugs, enzyme replacement therapy, gene preparations and anti-neoplastic drugs. As a result, diseased brain tissue often cannot receive the necessary amounts of therapeutic agents to properly heal. It is therefore generally desirable to temporarily “open” the BBB to permit therapeutic quantities of these agents to access the brain tissue in a safe, controlled and reversible manner, i.e., without damaging the brain tissue or its blood vessels and without permitting access permanently or for an extended period of time.
In brain tumors, the BBB inside the tumor (the “blood-tumor barrier,” or “BTB”) frequently exhibits a greater degree of permeability than the BBB located elsewhere in the brain. This is due to fenestrations in the tumor's endothelial cell layer and possibly also due to endothelial cell death in the tumor. Despite its relatively greater permeability, the BTB rarely permits sufficient amounts of therapeutic agents to be transported from the blood to the cancerous brain tissue. Like the BBB, the BTB also suffers the problem of inhibiting delivery of therapeutic agents. Thus, in discussing the delivery of therapeutic quantities of drugs for the treatment of brain cancer as set forth herein, the “BBB” will be broadly construed to include the blood-tumor barrier or “BTB.”
Previous attempts have been made to penetrate the BBB with therapeutic agents. In particular, prior research indicates that BBB permeability may be increased by (i) infusing hyperosmolaric solutions, such as mannitol, in close proximity to the BBB; (ii) administering drugs, such as bradykinin, intra-venously or intra-arterially; or (iii) disrupting the BBB by delivering focused energy to it. Each of these prior techniques suffers various disadvantages that limit its practical application.
Mannitol
Osmotic substances, most notably mannitol, have been used for decades to increase BBB permeability for drugs like methotrexate, carboplatin, and others. A catheter is placed into the internal carotid artery of a patient on the side where BBB disruption is intended. Mannitol solution is then rapidly infused, frequently followed by an intra-arterial injection of a drug. This method has demonstrated that an unspecific disruption of the BBB can be used to deliver the drug to the brain. See Neuwelt et al, Blood-Brain Barrier Disruption in the Treatment of Brain Tumors: Clinical Implications, in Implications of the Blood-Brain Barrier and Its Manipulation, Neuwelt, Editor: p. 195-253 (1989).
Although a positive effect on BBB permeability has been shown by such treatments, use of osmotic substances has not been widely adopted. The necessity for placement of an intra-arterial catheter prior to each drug treatment is cumbersome. Further, the mannitol injection is often associated with seizures and sometimes hemorrhages. Moreover, the resultant BBB opening occurs in the whole hemisphere treated, and it is difficult to control how long the BBB opening will persist.
Bradykinin
Other attempts to open the BBB with bradykinin or analogous substances showed early promising results. See, for example, Kroll et al, Improving drug delivery to intra-cerebral tumor and surrounding brain in a rodent model: a comparison of osmotic versus bradykinin modification of the blood-brain and/or blood-tumor barriers, Neurosurgery 43(4): p. 879-86 (1998). In these studies, the effect of bradykinin appeared to be mostly limited to the BTB. Specifically, bradykinin drugs increased the permeability of the BTB, with the additional advantage of permitting therapeutic drugs to be administered relatively easily via intra-venous injection.
Despite its above-noted advantages, the use of bradykinin drugs has some significant drawbacks that renders it impractical for improving drug delivery through the BBB. For instance, the effect of bradykinin is not targeted to a specific region of the BBB and may inadvertently expose healthy brain tissue to potentially noxious substances. Specifically, bradykinin drugs may increase the permeability of a patient's entire BBB even though the therapeutic drug delivery is intended for only a specific region of the brain (e.g., a brain tumor). This overall increase in BBB permeability has the undesired effect of increased exposure of healthy brain tissue to various toxins circulating in the blood.
Blocking Transporter Proteins
If a therapeutic drug is a substrate of a specific ATP Binding Cassette (“ABC”) transporter protein, inhibition of that transporter often enables the drug to move more easily across the BBB. Thus, transporter inhibition can be used to selectively increase the permeability of the BBB for the therapeutic drug. P-glycoprotein (“PGP”), for example, has a large number of known substrates, such as all glucocorticoids, doxorubicin, HIV protease inhibitors, phenytoin, taxol, and many others. It is apparent that treatment of a wide range of diseases is affected by just this one transporter protein. In fact, recent studies indicate that a substance blocking PGP could be used to enhance delivery of the drug paclitaxel across the BBB. See Fellner et al, Transport of paclitaxel (Taxol) across the blood-brain barrier in vitro and in vivo, J. Clin. Invest. 110(9): p. 1309-18 (2002).
Although blocking certain ABC transporter proteins, such as PGP, can reduce the vulnerability of the brain caused by a generalized increase of BBB permeability, this method is currently limited in its clinical application. For instance, because all substances that are substrates of the blocked transporter are permitted to pass through the BBB, some potentially dangerous substrates inadvertently may be able to penetrate the BBB. Further, ABC transporters may be expressed in other tissues besides the BBB. For example, PGP can also be found in the intestine, kidneys, gonads, placenta, hepatocytes, leucocytes and adrenal glands. Blocking PGP will therefore have a systemic effect and may, among other side-effects, cause an increased toxicity of its substrates. Another limitation of this approach is that drugs that are not a transporter substrate could not be delivered using this method. In summary, the PGP blocking method is limited because of its inability to deliver a wide variety of drugs to specific regions of the brain.
High Intensity Focused Ultrasound
Researchers have investigated the use of high intensity focused ultrasound (“HIFU”) to selectively disrupt the BBB for the purpose of transporting therapeutic agents to the brain. Ultrasound having a frequency equal to or greater than 1 Megahertz (MHz) can be precisely focused to a volume as small as one cubic millimeter. By concentrating the high-intensity ultrasonic energy on a relatively small region of the BBB, it is believed that the permeability of the exposed region is improved as a result of inertial cavitation and heating effects in the cerebral vasculature, i.e., the focused ultrasound beam may nucleate or otherwise enhance microbubble development along the luminal membrane of the blood vessels' lining. The applied HIFU energy causes these microbubbles to oscillate violently until they collapse (i.e., cavitate), thereby opening the BBB.
The region of the BBB affected by the HIFU insonation will typically exhibit increased permeability for extended periods of time; indeed, periods of increased permeability of 6 hours and, in some cases, up to 72 hours have been reported. See, for example, Hynynen et al., Noninvasive MR imaging-guided focal opening of the blood-brain barrier in rabbits, Radiology 220(3): p. 640-6 (2001), and Mesiwala et al., High-Intensity Focused Ultrasound Selectively Disrupts the Blood-Brain Barrier In Vivo, Ultrasound in Med. & Biol. 28(3): p. 389-400 (2002). This relatively long duration for which the affected region of the BBB remains permeable leaves the brain vulnerable to toxins from the blood and also increases the risk of brain edema.
Further, the application of high-intensity focused acoustic energy can also cause permanent biological damage in and around the BBB. For instance, the process of ablating portions of the BBB may cause, e.g., cell death or cell necrosis. HIFU also may be destructive to osseous tissue (bone) surrounding the insonicated region of the BBB. Specifically, the bone absorbs the ultrasound energy which, in turn, creates localized heating in the bone. In general, the higher the ultrasound frequency, the more pronounced is the energy absorption and the resulting temperature increase in the bone. To prevent heating of the bone during a HIFU procedure, a craniotomy may have to be conducted and part of the skull removed to safely apply the HIFU energy directly to the brain.
In addition to the above-noted biological dangers associated with HIFU, implementation of HIFU is impractical for several additional reasons. HIFU is applied to a relatively small volume (˜1 mm3) and is not well suited for treatment of larger volumes, e.g., as required in tumor therapy. Accordingly, to create sufficient disruption of the BBB in and around a tumor, a multitude of HIFU exposures typically must be applied to a series of adjacent, non-overlapping target volumes. This tends to be a very difficult and time-consuming process. To facilitate the process, HIFU is often coupled with image guidance systems, such as magnetic resonance imaging guidance, to ensure that the focused ultrasound energy is precisely targeted in the desired BBB regions. The necessity for image-guidance places yet a further constraint on the practical application of HIFU. For instance, HIFU may not be able to effectively treat a patient when the spread of diseased tissue cannot be fully visualized using current imaging technology.
Practical implementation of HIFU treatment is also limited by the instrumentation needed to apply HIFU. HIFU systems usually employ a complex phased array of ultrasound transducers, powered and controlled by a computer controller. The phased array applies focused ultrasound to those locations in the BBB targeted for treatment. These focused locations can be adjusted electronically using different relative phase excitations of the array elements. Alternatively, a concave surface transducer may be used to focus the ultrasound beam. In either case, the HIFU system typically comprises complex electronics which require sophisticated configuration and maintenance. As such, the HIFU system may require operators to consume substantial amounts of time and resources to maintain proper operation of the system.
Some researchers have experimented with exogenous agents, such as microbubble agents, in an attempt to improve the effectiveness of HIFU for opening the BBB. However, Karshafian et al. at the University of Toronto showed that the presence of an exogenous microbubble agent not only enhances the BBB permeability but also increases the amount of cell death resulting from the HIFU exposure, especially at lower ultrasonic frequencies. Thus, introduction of the microbubble agent does not appear to remedy the deficiencies of HIFU, and in some cases may even render application of HIFU more dangerous. Also, the combination of three parameters—agent, drug and ultrasound energy—will require extensive validation of the combinations of doses of agent, drug and energy in order to ensure safety and effectiveness. This may represent a prohibitively costly barrier to prove feasibility of the method.
A new method to reversibly increase the permeability of the BBB is needed. Unlike current approaches, this method should be non-invasive to allow its wide-spread use. Regional selectivity with regard to where BBB disruption will occur is desirable. Further, the method should affect a volume of brain tissue large enough to include a complete tumor as well as its surrounding tissue.